Oscillators that have a frequency determining crystal are commonly used to provide a stable output frequency. However, the crystals used in these oscillators are temperature sensitive and are therefore required to be temperature compensated to maintain a stable oscillator output frequency.
A temperature compensated crystal oscillator (TCXO) generally includes a temperature compensation circuit which generates a temperature variable output voltage which is applied to a frequency control input to stabilize the oscillator output frequency over temperature. To improve phase noise prior art TCXOs use a low-pass filter with a large time constant to remove unwanted noise generated at the output voltage. Unfortunately, when power is turned on to the TCXO, the low-pass filter takes a long time to settle and, consequently, the oscillator takes a long time to stabilize to the correct temperature compensated output frequency.
Typically, prior art TCXOs use a low-pass filter with a twenty millisecond time constant to sufficiently attenuate output noise from the temperature compensation circuit. At power on, these TCXOs require about ten to twelve milliseconds for their output frequencies to settle to within specifications. This startup delay is unacceptable for modern communication systems, including cellular telephones and two-way radios, which require low-power crystal oscillators that provide a stable output frequency with low phase noise almost immediately after turn-on. In particular, many cellular radios which use a discontinuous receive mode require startup delays of less than three milliseconds.
Integrated circuit (IC) based TCXOs have been unable to achieve three millisecond startup times. TCXOs made up of discrete components have fast startup times, but are relatively large and expensive. In addition, their phase noise degrades with temperature.
There is a need for a low power TCXO that has a fast startup time with improved phase noise. There is also a need for a TCXO that retains the advantages of IC based TCXOs with the performance of discrete component TCXOs.